Super high frequency transmitting and receiving antennas include a parabolic-type antenna, a microstrip antenna, a waveguide slot array antenna, and so forth. Among these antennas, a microstrip array antenna or a waveguide slot array antenna is mainly used for miniaturization through thickness reduction.
The microstrip array antenna has a microstrip patch array structure using a dielectric substrate, in which a loss of a transmitted or received signal is large depending on a loss coefficient of a dielectric based on characteristics of the dielectric substrate, and an ohmic loss of a conductor occurs, and the loss increases especially for a higher frequency, such that the use of the microstrip array antenna is avoided in a super high frequency band.
The waveguide slot array antenna has a structure in which a hole in the form of a slot is formed in a general waveguide, without using the dielectric substrate. Generally, a waveguide is a hollow metal pipe and a sort of high pass filter in which a guided mode has a specific cutoff frequency and a dominant mode is determined by a size of the waveguide. The waveguide has lower attenuation than a parallel two-wire line, a coaxial cable, etc., and thus is mostly used for high power in a microwave transmission line. The waveguide may have various cross-sectional shapes, depending on which the waveguide is classified into a circular waveguide, a quadrangular waveguide, an oval waveguide, and so forth.
Techniques related to the waveguide slot array antenna are disclosed in, for example, a Korean Patent Application No. 2006-18147 (entitled “Stacked Slot Array Antenna”, filed by MOTONICS Co., Ltd. on Feb. 24, 2006 and invented by Taekwan Cho, et al.) or a Korean Patent Application No. 2007-7000182 (entitled “Planar Antenna Module, Triple Plate-Type Planar Array Antenna, and Triple Plate Line-Waveguide Converter”, filed by Hitachi Chemical Company, Ltd., on Jan. 4, 2007, and invented by Oota Masahiko et al.).
FIG. 1A is a perspective view in which each layer of a conventional waveguide slot array antenna having a stacked multi-layer structure is partially cut. Referring to FIG. 1A, the conventional waveguide slot array antenna includes a feeding plate 11 in which an input feeding slot 112 is formed, a distribution plate 12 which is installed on the feeding plate 11 and in which a distributor and coupling slots 122 are formed, a main radiation plate 13 which is installed on the distribution plate 12 and in which a cavity structure and an excitation slot (or a radiation slot) 132 are formed, and an auxiliary radiation plate 14 which is installed on the main radiation plate 13 and in which a polarization slot 142 is formed to generate a polarized wave having a polarization plane inclined at 45 degrees (°).
Once a signal is input from the feeding slot 112 of the feeding plate 11, the input signal is distributed, for example, in an equal ratio, through the distribution plate 12, and each distributed signal is delivered to each cavity formed in the main radiation plate 13 through the coupling slots 122. The signal delivered to the cavity of the main radiation plate 13 is distributed and radiated in an equal ratio through, for example, four excitation slots 132 formed for each cavity. The excitation slots 132 are arranged to have a preset interval and preset arrangement therebetween according to an operating frequency.
In the auxiliary radiation plate 14 installed on the main radiation plate 13, the polarization slots 142, each of which one-to-one corresponds to each excitation slot 132 of the main radiation plate 13, is formed, and the signal delivered to the polarization slot 142 is rotated at 45 degrees when compared to radiation from the excitation slot 132 and is radiated to the space. That is, a wave polarized at 45 degrees in a vertical/horizontal direction is generated by the auxiliary radiation plate 14. Referring to a slot shape of the excitation slot 132, the excitation slot 132 has, for example, an approximately rectangular shape, and is formed in an erect position or posture in the vertical/horizontal direction, and for a slot shape of the polarization slot 142, the polarization slot 142 has a rectangular shape similar to the approximately rectangular shape of the excitation slot 132, but when compared to the slot shape of the excitation slot 132, the rectangular shape of the polarization slot 142 is formed in a position or posture mechanically rotated at 45 degrees in the vertical/horizontal direction and thus may be globally similar to a diamond shape. Such a structure may be regarded as a structure that forms one radiation slot by a combination of the excitation slot 132 and the polarization slot 142.
As such, to operate the conventional waveguide slot array antenna for vertical/horizontal polarization, the auxiliary radiation plate 14 is used and the polarization slot 142 of the auxiliary radiation plate 14 may have a rectangular shape rotated at 45 degrees with respect to the excitation slot 132 to rotate a polarization plane of a signal radiated from the excitation slot 132 at 45 degrees. With this structure, a side lobe component is significantly suppressed by a total length of a vertical/horizontal plane.
However, as the rectangular polarization slot 142 formed in the auxiliary radiation plate 14 is rotated at 45 degrees in the vertical/horizontal direction to have a shape similar to the diamond shape, an arrangement interval between the polarization slots 142 on the vertical/horizontal plane may fail to satisfy a proper distance criterion required when a wavelength of an operating frequency is considered. That is, as indicated by a in FIG. 1A, a distance, especially between the polarization slots 142 arranged diagonally to each other may increase. Such a structure may cause a grating lobe.
More specifically, in an array antenna, if a distance between arrays exceeds one wavelength, a specific radiation angle is produced at which signals radiated from respective radiation slots are in phase. A lobe produced in this case is called a grating lobe that is a sort of main lobe. The grating lobe is generated by a phase of an array element in the array antenna, and the phase is controlled by a distance between elements.
FIG. 1B shows a state in which a main lobe and a grating lobe are produced, for example, in positions P1 and P2 of two polarization slots located diagonally (having a distance of d therebetween) in FIG. 1A. Referring to FIG. 1B, when a phase difference between two paths is one wavelength λ, the grating lobe is produced at an angle rotated by θ from the main lobe. The generated angle may be simply expressed with the following equation.
  θ  =            sin              -        1              ⁡          (              λ        d            )      
Due to the grating lobe, radiation pattern envelope (RPE) standards prescribed in corresponding countries may not be satisfied. Thus, a scheme for suppressing the grating lobe is required.
It may be possible to suppress the grating lobe by disposing multiple excitation slots on an identical antenna area where an arrangement interval between excitation slots is reduced, but in a conventional structure, the number of excitation slot arrays increases to a power of 2 depending on a distribution plate and a cavity structure that distributes a signal on a main radiation plate, showing some limitations in the design of arrangement of excitation slots.